Browsing a large graph, such as ontology graph, is a challenge because of its size. One well-known technique is based upon directed graph drawing and a layout schema, and is referred to as Sugiyama's scheme (STT).
One feature of STT that researchers find attractive is its ability to provide well organized graphs with labels. However, when dealing with graphs that are too large, very few of the graph's nodes can be displayed on a rendering surface, whereby the viewer can quickly become disoriented within the graph.
Moreover, STT imposes a significant performance penalty. For example, when dealing with a number of nodes and links each on the order of thousands, contemporary computer systems need over a minute to perform STT layout; one example of 2827 nodes and 4734 links took approximately one-and-a half minutes to perform layout. The time tends to increase exponentially as nodes and links increase. STT is thus not suitable for a large ontology graph with thousands or tens of thousands of nodes and links.